Endpoint window with controlled texture surface

ABSTRACT

A chemical mechanical polishing pad window having a controlled texture surface comprising repeated patterned features. The window results in an improved endpoint detection and in situ rate monitoring by providing consistent values of the ISRM max-min  characteristic over the lifetime of a CMP pad. Also provided is a chemical mechanical polishing pad with the inventive window.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority under 35 U.S.C. § 119 of U.S. Provisional Application Ser. No. 63/141,368, filed Jan. 25, 2021, entitled “ENDPOINT WINDOW WITH CONTROLLED TEXTURE SURFACE,” which is hereby incorporated by reference.

TECHNICAL FIELD

This disclosure generally relates to chemical mechanical planarization, and more specifically to an endpoint window with a controlled texture surface.

BACKGROUND

An integrated circuit is typically formed on a substrate by the sequential deposition of conductive, semi-conductive, and/or insulative layers on a silicon wafer. A variety of fabrication processes require planarization of at least one of these layers on the substrate. For example, for certain applications (e.g., polishing of a metal layer to form vias, plugs, and lines in the trenches of a patterned layer), an overlying layer is planarized until the top surface of a patterned layer is exposed. In other applications (e.g., planarization of a dielectric layer for photolithography), an overlying layer is polished until a desired thickness remains over the underlying layer. Chemical mechanical planarization (CMP), which is sometimes alternatively referred to as chemical mechanical polishing, is one method of planarization. This planarization method typically involves a substrate being mounted on a carrier head. The exposed surface of the substrate is typically placed against a polishing pad on a rotating platen. The carrier head provides a controllable load (e.g., an applied force) on the substrate to push it against the rotating polishing pad. A polishing liquid, such as slurry with abrasive particles, can also be disposed on the surface of the polishing pad during polishing.

In some cases, a CMP pad may include a window to enable in situ monitoring of the polishing process. For example, a laser light may be passed through the window, reflected off of the material being removed via CMP, and the intensity of the reflected light may be used to determine when the etching process is complete. When a reflective material (e.g., a metal) is removed, the amount of light reflected from the wafer decreases. This decrease is observed by monitoring the intensity of the laser light returning through the window over time and can be used to detect the endpoint of an etching process (i.e., to determine when the reflective material is completely removed from the wafer's surface).

SUMMARY

This disclosure recognizes that reliable CMP endpoint detection requires a window with a finely tuned initial percent transmission to laser light and that this transmission should remain as stable as possible throughout the life of the pad. Previous windows for CMP pads fail to satisfy requirements for accurately and reliably monitoring polishing processes, resulting in process inefficiencies and decreased yields. Previous windows in CMP pads are susceptible to significant changes in transmission during use. For example, the diamond conditioner used as part of the CMP workflow may scratch and/or remove a portion of the window surface, thereby altering the transmission of light through the window. This disclosure recognizes that when the window is thinned after continued use and/or conditioning, the window is more likely to deform during use, resulting in undesired measurement variability, which may be quantified by an increased ISRM_(max-min) characteristic (see FIG. 6C and TABLE 1 and the corresponding descriptions below). In some cases, the surface of the wafer may be scanned to determine if a reflective material is removed from specific regions of the wafer. Previous windows may cause variability in the intensity of light measured at different regions of a wafer's surface even when material has not been removed (see FIG. 1C and corresponding description below). This disclosure also recognizes not only that light transmission should be less than a threshold level that is determined based on properties of the detector(s) used for CMP monitoring and endpoint detection but also that the presence of a repeating pattern on a surface of the window decreases measurement variability over time, resulting in a decreased and more consistent value of the ISRM_(max-min) characteristic. If light transmission is too high, the detector(s) may become saturated, and signal may be unstable and/or unreliable. However, even if a window provides a sufficiently low light transmission, this transmission property on its own is not generally sufficient for providing reliable measurements (e.g., consistent values of the ISRM_(max-min) characteristic) over the lifetime of a CMP pad.

This disclosure provides an improved window for endpoint detection and in situ rate monitoring. This unique CMP pad window provides a solution to problems of previous CMP pad window technologies including those described above. The CMP pad window described in this disclosure has a controlled texture surface comprising repeated patterned features, for example, a unique texture on at least one surface of the window. For example, a controlled texture surface comprising repeated patterned features (e.g. micropatterned texture) may be provided on the bottom surface of a CMP pad window to diffuse light passing therethrough. The repeating pattern provides a light diffusing texture. The repeated patterned features can be prepared using injection molding, laser cutting, and/or any other machining or surface patterning technique(s). The new CMP pad window provides reliable and reproducible removal rate monitoring and endpoint detection. This disclosure also includes new methods for preparing a surface texture on materials used for CMP pad windows. These methods facilitate more precise tuning of window texture and the transmission of light through the window, both of which are critical for providing reliable and improved performance. Methods may involve injection molding, machining (e.g., CNC machining with an appropriate diameter endmill bit, laser machining), and/or any other appropriate technique.

In an embodiment, a window for a chemical mechanical planarization (CMP) pad includes (e.g., is formed of) a material that is transmissive to light. A first surface of the window has repeated patterned texture or features. The first surface may correspond to (e.g., face the same direction as) a bottom surface of the CMP pad which does not contact a substrate being planarized during a CMP process using the CMP pad.

A measured ISRM_(max-min) characteristic of the window may be less than a threshold value (e.g., 1%, 0.5%, or 0.3%). The ISRM_(max-min) characteristic may be a difference between a maximum percent intensity and a minimum percent intensity measured across a surface of a wafer comprising a reflective material. The ISRM_(max-min) characteristic of the window may change by less than the threshold amount (e.g., 5%, 10%, 25%, or 50%) following use of the CMP pad for chemical mechanical planarization of one or more wafers for a period of time (e.g., at an end of a useful lifetime of the CMP pad).

The repeated patterned features are configured to diffuse light passing through the window. The repeated patterned features may include regularly spaced raised features. The window may have a width and length that is less than the width. The width and the length may characterize physical dimensions of the window (e.g., whether the window has a rectangular, rounded rectangular, ovular, or any other shape). As one example, the repeated patterned features may include a first set of regularly spaced raised features parallel to a direction of the width of the window and a second set of regularly spaced raised lines at an angle (e.g., in a range from 20° to 60°) relative to the first set of regularly spaced raised features. As another example, the repeated patterned features may include a crosshatch texture with a first set of regularly spaced features in the first surface at a first angle (e.g., 45°) relative to a direction of the width of the window and a second set of regularly spaced features in the first surface at a second angle (e.g., 90°) relative to the first set of lines, wherein the first angle is different than the second angle.

In another embodiment, a chemical mechanical planarization (CMP) pad includes a top surface which contacts a substrate being planarized during a CMP process using the CMP pad, a bottom surface opposite the top surface, and a window that allows light to pass between a top side associated with the top surface and a bottom side associated with the bottom surface. The window includes a material transmissive to the light. A first surface of the window has repeated patterned texture or features. The first surface may correspond to (e.g., face the same direction as) the bottom surface of the CMP pad which does not contact a substrate being planarized during a CMP process using the CMP pad.

A measured ISRM_(max-min) characteristic of the window may be less than a threshold value (e.g., 1%, 0.5%, or 0.3%). The ISRM_(max-min) characteristic may be a difference between a maximum percent intensity and a minimum percent intensity measured across a surface of a wafer comprising a reflective material. The ISRM_(max-min) characteristic of the window may change by less than the threshold amount (e.g., 5%, 10%, 25%, or 50%) following use of the CMP pad for chemical mechanical planarization of one or more wafers for a period of time (e.g., at an end of a useful lifetime of the CMP pad).

The repeated patterned texture or features are configured to diffuse light passing through the window. The repeated patterned texture may include regularly spaced raised features. The window may have a width and length that is less than the width. The width and the length may characterize physical dimensions of the window (e.g., whether the window has a rectangular, rounded rectangular, ovular, or any other shape). As one example, the repeated patterned features may include a first set of regularly spaced raised features parallel to a direction of the width of the window and a second set of regularly spaced raised lines at an angle (e.g., in a range from 20° to) 60° relative to the first set of regularly spaced raised features. As another example, the repeated patterned features may include a crosshatch texture with a first set of regularly spaced features in the first surface at a first angle (e.g., 45°) relative to a direction of the width of the window and a second set of regularly spaced features in the first surface at a second angle (e.g., 90°) relative to the first set of lines, wherein the first angle is different than the second angle.

BRIEF DESCRIPTION OF FIGURES

To assist in understanding the present disclosure, reference is now made to the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1A is a diagram of an example system for chemical mechanical planarization (CMP);

FIG. 1B is a diagram of a portion of the system illustrated in FIG. 1A that is used for in situ rate monitoring and endpoint detection;

FIG. 1C shows example plots of detected intensity versus wafer diameter during example CMP processes using different CMP pad windows;

FIG. 2 is a diagram of an example CMP pad including a window and an expanded view of the texture surfaces of the window;

FIGS. 3A, 3B, and 3C are diagrams depicting example surface textures for CMP pad windows;

FIG. 3D is a diagram of a cross-sectional view of a randomly textured surface at a high magnification;

FIGS. 3E, 3F, and 3G are diagrams illustrating example cross-sectional views of patterned textures of a CMP window at a high magnification;

FIG. 4 is a diagram depicting the combination of two pieces with textured surfaces to form an example window for a CMP pad;

FIGS. 5A and 5B are images of example molds used for injection molding to prepare windows with textured surfaces;

FIGS. 6A and 6B are plots of percent transmission of light at different wavelengths for different windows for CMP pads;

FIG. 6C is a bar graph showing values of the ISRM_(max-min) characteristic of example CMP windows after CMP pad break-in and at the end of pad life; and

FIGS. 7, 8, and 9 are flowcharts of example processes for preparing CMP pad windows and CMP pads comprising such windows.

DETAILED DESCRIPTION

It should be understood at the outset that, although example implementations of embodiments of the disclosure are illustrated below, the present disclosure may be implemented using any number of techniques, whether currently known or not. The present disclosure should in no way be limited to the example implementations, drawings, and techniques illustrated below. Additionally, the drawings are not necessarily drawn to scale.

Chemical Mechanical Planarization System with Endpoint Detection

FIG. 1 illustrates a system 100 for performing chemical mechanical planarization. System 100 includes a CMP pad 200 (also referred to as a “polishing pad,” see also FIG. 2 and the corresponding description below) which is placed on or attached to a platen 102. For example, an adhesive layer (not shown) may be used to attach the polishing pad to the platen 102. In some cases, the platen 102 may be rotated during chemical mechanical planarization. A wafer 104 (e.g., a silicon wafer with or without conductive, semi-conductive, and/or insulative layers, as described above) is attached to a head 106 of a rotatable chuck. The wafer 104 may be attached using vacuum and/or a reversible adhesive (e.g., an adhesive that holds the wafer 104 in place during chemical mechanical planarization but allows the wafer 104 to be removed from the head 106 after chemical mechanical planarization). As illustrated in FIG. 1, a pressure may be applied to the wafer 104 during chemical mechanical planarization (e.g., to facilitate contact between the surface of the wafer 104 and the polishing pad 200).

An example polishing pad 200 is illustrated in FIG. 2 and described in greater detail below. In brief, the polishing pad 200 generally has a circular or approximately cylindrical shape (i.e., with a top surface, a bottom surface, and a curved edge). The polishing pad 200 may comprise polyurethane. The polishing pad 200 includes a window 202. The window 202 may be any appropriate dimensions for a given application. The window 202 may have specially designed textures on its top and bottom surfaces as described in greater detail below with respect to FIGS. 2-4. For instance, a window 202 may have a first texture on one surface and the same or a different texture on the other surface. In some embodiments, the window 202 may have repeated patterned features and/or roughened texture on one surface and a repeated patterned texture on the other surface (see FIGS. 3A-C and corresponding description below). In some embodiments, the window 202 has a roughened texture on the top surface and a repeated patterned texture on the bottom surface (see examples of FIGS. 3A and 3B). In some embodiments, the window 202 has a crosshatch pattern machined in both the top surface and the bottom surface (see example crosshatch pattern illustrated in FIG. 3C).

The window 202 is generally disposed in or over an aperture in the CMP pad 200. As illustrated in the side-view depiction of FIG. 2B, light from laser 112 may be directed to pass through the window 202. A portion of the light reflected from the reflective material 118 on the surface of the wafer 104 being polished passes back through the window 202 and reaches a detector 114. A signal from the detector 114 is provided to a computer 116, which is configured to monitor the light intensity over time. The computer 116 may be configured to detect that an endpoint is reached when the intensity reaches a predefined value (e.g., or a user may monitor the plot 150 to determine when the CMP process is complete).

FIG. 1C shows an example plot 150 of light intensity information as a function of the wafer diameter when the wafer 104 is scanned over the window 202. Advanced microfabrication processes may require a polishing process to be monitored as a function of position along the surface of the wafer 104. These surface-scanning approaches may be referred to as in situ rate monitoring (ISRM). For example, a different pressure may be applied during polishing of certain regions of the wafer's surface, and the pressure may be determined based on information obtained through in situ rate monitoring. Reliable performance of such advanced polishing processes requires a consistent and reliable signal as a function of position along the surface of the wafer 104. If the window 202 is functioning properly (e.g., as is particularly facilitated using the uniquely textured window 202 described in this disclosure), a uniform intensity profile 152 a is seen initially and decreases to a lower value 156 a following completion of the CMP process.

The difference 154 a between the maximum and minimum intensity along the surface of the wafer 104 is a relatively small value for the new window 202 described in this disclosure. For example, the difference 154 a for windows 202 described in this disclosure may be less than a threshold value of about 1%, 0.5%, 0.1%, or the smaller (in units of percent light transmission). This difference 154 a may be stable throughout the life cycle of the window 202 (see FIG. 6C and TABLE 1 below). This difference 154 a is referred to herein as an in situ rate monitoring min/max characteristic (ISRM_(max-min) characteristic). This relatively small variation (i.e., based on small difference 154 a) facilitates the use of advanced processing techniques to actively control the polishing removal rate in different zones or regions of the wafer's surface. In contrast, a previous window may have an inconsistent intensity profile 152 b which decreases over time during polishing (see profile 156 b after polishing is complete for the previous CMP pad window). The difference 154 b between the maximum and minimum intensity along the surface of the wafer 104 is a relatively large value for the previous window 202, such that advanced CMP processes cannot reliably be performed.

Returning to FIG. 1A, polishing pad 200 may have any appropriate thickness and any appropriate diameter (e.g., to be employed with a CMP system such as system 100, described above). For instance, the thickness of a polishing pad 200 may be in a range from less than or about 0.5 millimeters (mm) to greater than 5 centimeters (cm). In some embodiments, the thickness of the polishing pad 200 may be in a range from 1 mm to 5 mm. Polishing pad diameter is generally selected to match or be just smaller than, the diameter of the platen 102 of the polishing system 100 used. The polishing pad 200 generally has a uniform or near-uniform thickness (e.g., a thickness that varies by no more than 50%, 25%, 20%, 10%, 5%, or less across the radial extent of the polishing pad).

A slurry 108 may be provided on the surface of the polishing pad 200 before and/or during chemical mechanical planarization. The slurry 108 may be any appropriate slurry for planarization of the wafer type and/or layer material to be planarized (e.g., to remove a silicon oxide layer from the surface of the wafer 104). The slurry 108 generally includes a fluid and abrasive and/or chemically reactive particles. Any appropriate slurry 108 may be used. For example, the slurry 108 may react with one or more materials being removed from a surface being planarized. A conditioner 110 is a device which is configured to condition the surface of the polishing pad 200. The conditioner 110 generally contacts the surface of the polishing pad 200 and removes a portion of the top layer of the polishing pad 200 to improve its performance during chemical mechanical planarization. For example, the conditioner 110 may roughen the surface of the polishing pad 200.

As described in greater detail below, the uniquely textured window 202 described in this disclosure reduces or eliminates changes in the transmission of light through the window following contact with the slurry 108, the wafer 104, and/or the conditioner 110. For example, a bottom surface of the window 202 may have a repeated patterned feature, such as the repeating patterned textures illustrated in FIGS. 6E-G and described in greater detail below. This repeated patterned feature may facilitate consistent measurements and low values of the ISRM_(max-min) characteristic throughout the life of the CMP pad 200. For example, even as the CMP pad 200 and window 202 become thinner during use, resulting in possible movement and/or bending of the window 202, the ISRM_(max-min) characteristic remains consistent with the new window 202 of this disclosure (see also TABLE 1 and FIG. 6C and the corresponding descriptions below). As another example, a texture of the top surface of the window 202 (i.e., the surface that exposed during use of the CMP pad 200) may have a texture that mimics that generated by contact with the conditioner 110, slurry 108, and/or wafer 104.

Example Polishing Pad with Improved window

FIG. 2 illustrates an example CMP pad 200 in greater detail. The CMP pad 200 includes a top surface 204, which contacts a material during a CMP process, a bottom surface 206, and a window 202 on or within an aperture in the CMP pad 200. The CMP pad 200 generally has a circular or approximately cylindrical shape. The thickness 208 of the CMP pad 200 may be in a range from about 1 mm to about 10 mm or more. The diameter of the CMP pad 200 may be in a range from about 500 mm to about 800 mm. The CMP pad 200 generally has a uniform thickness 208. A uniform thickness is defined as a thickness that varies by no more than 50%, 25%, 20%, 10%, 5%, or less across the radial extent of the pad 200. In other words, the thickness measured near the center of the pad 200 is substantially the same as the thickness near the edge of the pad 200.

The CMP pad 200 may be formed of a thermoset material, such as a polyurethane, or any other appropriate material. The top surface 204 may include grooves or any other appropriate structure or pattern for facilitating CMP (see expanded view 210). For instance, grooves may facilitate the transport of polished material and/or any other products of the CMP process away from the surface 204 of the CMP pad 200 and the wafer 104 being planarized. The CMP pad 200 includes a window 202 on or in an aperture of the CMP pad 200. The window 202 may be formed of a thermoset or a thermoplastic material, such as a polyurethane, polyethylene terephthalate glycol, a cyclic olefin copolymer, or any other appropriate material (i.e., a material that is appropriately transmissive to light (see FIGS. 6A and 6B) and moldable/machinable for preparing textured surfaces 220, 222). The window 202 may be located at any appropriate position in the CMP pad 200. The window 202 may be any appropriate size for viewing a portion of a wafer 104 during a CMP process (see FIGS. 1A and 1B for reference).

An expanded top-side view 210 of the window 202 shows a top surface 220 of the window 202. The top surface 220 may have a unique texture, as described in greater detail with respect to FIGS. 3A and 3C below. For example, the top surface 220 may have a roughened texture (see FIGS. 3A and 3D). The roughened texture may be achieved through a random roughening process, such as sanding, treatment with a conditioner (see FIG. 1A), sand blasting, or the like. The roughened texture may correspond to an average surface roughness in a range from 1 to 50 micrometers, or in some cases, in a range from about 2 to 10 micrometers. As another example, the top surface 220 may have a repeated patterned texture, such as a crosshatched patterned texture (see, e.g., FIGS. 3C, F, G), a “wavy” patterned texture (see, e.g., FIG. 3E), or any other appropriate texture.

An expanded bottom-side view 212 of the window 202 shows a bottom surface 222 of the window 202. The bottom surface 222 may have a controlled texture comprising repeated patterned features for diffusing light passing through the window 202. For example, the bottom surface 222 may have a texture that includes repeated patterned features and optionally a roughened texture (see FIG. 3B). For example, the bottom surface 222 may have a crosshatched patterned feature (see FIG. 3C), a “wavy” patterned texture, or any other appropriate controlled texture. Further examples of patterned textures are illustrated in FIGS. 3D-G and described below. The texture of the top surface 220 may be the same as or different than the texture of the bottom surface 222. While the example of FIG. 2 describes the top surface 220 of the window 202 as corresponding to (i.e., facing the same direction as) the top surface 204 of the CMP pad 200 and the bottom surface 222 of the window 202 as corresponding to (i.e., facing the same direction as) the bottom surface 206 of the CMP pad 200, it should be understood that the direction of the window 202 can be changed. For instance, the orientation of the window 202 can be reversed such that the top surface 202 of the window 202 faces the direction of the bottom surface 206 of the CMP pad 200.

FIG. 3A illustrates an example top surface 220 of a window 202. In the example of FIG. 3A, the top surface 220 has a roughened texture 302. The roughened texture may have an average roughness in a range from about 1 to 50 micrometers, or in some cases, in a range from about 2 to 10 micrometers. The example top surface 220 of FIG. 3A can be prepared using injection molding (see FIG. 5B), machining, treatment with an abrasive material, or any other appropriate process. In some cases, the use of injection molding may provide improved consistency between different window preparations at a relatively low manufacturing cost. For example, the texture 302 of the top surface 220 may be the randomly roughened texture 340 shown in FIG. 3D. The roughened texture 340 illustrates a portion of a cross-section through a randomly roughened surface, such as top surface 220 of FIG. 3A. The randomly roughened texture 340 has random features 342 that are randomly distributed on the surface (e.g., on top surface 220). The random features 342 may have a size 344 a,b (e.g., in width and/or height/depth as shown in FIG. 3D) in a range from about 1 to 50 micrometers, or in some cases, in a range from about 2 to 10 micrometers.

FIG. 3B depicts an example bottom surface 222 of a window 202. In the example of FIG. 3B, the bottom surface 222 may have a region with a controlled texture 304 and a region with a different texture 320. The region with texture 304 may be the area (with a width 316 and length 318) through which light from laser 112 passes during in situ rate monitoring/endpoint detection (see FIGS. 1A-C and corresponding description above). For instance, all or a portion of the region with controlled texture 304 may be aligned over the aperture of the CMP pad 200, such that light passes through this textured region. Texture 304 of the bottom surface 222 may include a repeated pattern designed to diffuse light passing through the window 202. Meanwhile, the remaining surface area with texture 320 may rest on or contact a portion of the pad 200 to aid in attachment of the window 202 to the pad 200. The length 318 and width 316 of the region with texture 304 may be any appropriate values for a given in situ rate monitoring/endpoint detection application. As a non-limiting example, the length 318 may be about 50 millimeters (e.g., 2 inches) and the width 314 may be 13 millimeters (0.5 inches).

In an example embodiment, the texture 304 through which light passes for in situ rate monitoring/endpoint detection includes a repeated pattern with regularly spaced features 306 (e.g., peaks of troughs/cups 352 of FIG. 3E or rounded ridges 362 of FIGS. 3F, G). The features 306 can be in straight lines as illustrated. In some cases, the features 306 may be curved or may have any other appropriate shape or design. The features 306 may be raised relative to the surface 222 or at a depth below the surface 222. More generally, the average height of the surface 222 may be raised, recessed, or approximately coplanar with features 306. For example, the features 306 may have a height or depth in a range from about 25 micrometers to about 500 micrometers (e.g., 0.001 inch to about 0.02 inch). The first set of features 306 may be parallel to the direction of the width 316 of the window 202, as shown in the example of FIG. 3B, or at any other appropriate angle (see FIG. 3C). The features 306 may have a width in a range from about 25 micrometers to about 500 micrometers (e.g., 0.001 inch to about 0.02 inch). The distance 308 between adjacent features 306 (e.g., the pitch of the repeated patterned features 306) may be in a range from about 120 micrometers to about 750 micrometers (e.g., 0.005 to about 0.03 inch).

FIG. 3E shows an example of controlled texture comprising repeated patterned features 350. The repeated patterned features 350 of FIG. 3E may be referred to as a “wavy” feature. FIG. 3E illustrates a portion of a cross-section through a surface with the repeated patterned features 350, such as bottom surface 222 of FIG. 3B or 3C. For example, the repeated patterned features 350 may be the texture 304 of FIG. 3B or the texture 322 of FIG. 3C. For example, features 352 may correspond to features 306, 310 of FIG. 3B or features 324, 330 of FIG. 3C. The repeated patterned feature 350 has regular features 352 that are troughs or cups formed on the surface (e.g., via machining or any other appropriate method. In some embodiments, the repeated patterned features 350 are also roughened, such that it also includes roughened features similar to those described with respect to FIG. 3D above (see also FIG. 3G showing an example of a combination of repeating patterned features and roughened texture). The width 354 of the repeated patterned features 350 may be any appropriate value. As an example, the width 354 may be in a range from about 0.1 millimeters to about 0.5 millimeters. The depth 356 of the features 352 may be any appropriate value. As an example, the depth 356 may be in a range from about 2 micrometers to about 100 micrometers.

FIGS. 3F and 3G show other example of controlled textures comprising repeated patterned features 360 and 370, respectively. The repeated patterned features 360 and 370 of FIGS. 3F and 3G may be referred to as rounded ridged features. FIGS. 3F and 3G illustrate a portion of a cross-section through a surface with the repeated patterned features 360 and 370, such as bottom surface 222 of FIG. 3B or 3C. For example, the repeated patterned features 360 and 370 may be the texture 304 of FIG. 3B or the texture 322 of FIG. 3C. For example, features 362 may correspond to features 306, 310 of FIG. 3B or features 324, 330 of FIG. 3C. The repeated patterned textures 360 and 370 have regular features 362 that are ridges formed on the surface (e.g., via injection molding or any other appropriate method). The texture 370 of FIG. 3G is also randomly roughened (e.g., the same as or similarly to as described with respect to FIG. 3D above). The width 364 of the features 362 may be any appropriate value. As an example, the width 364 may be in a range from about 0.1 millimeters to about 0.5 millimeters. The spacing 366 between adjacent features 362 may be any appropriate value. As an example, the spacing 366 may be in a range from about 0.1 millimeters to about 0.5 millimeters. The spacing 366 may be the same as or different than the width 364. The height 368 of the features 362 may be any appropriate value. As an example, the height 368 may be in a range from about 25 micrometers to about 500 micrometers.

An optional second set of features 310 may be at an angle 314 (e.g., 20° to) 90° relative to the first set of features 306. The second set of features 310 can be straight, curved, or any other appropriate shape or design, for example, as described with respect to FIGS. 3E-G above. For example, the angle 314 may be about 60°, or any appropriate angle. The repeated patterned features 310 may have the same or a different height/depth, width, and pitch 312 to those described above for features 306.

Still referring to FIG. 3B, texture 320 may be any texture. For instance, texture 320 may be a roughened texture that is the same as or similar to that described above with respect to FIG. 3A. In some cases, the texture 304 may include both the repeated patterned features 306, 310 and a roughened texture (see, e.g., FIG. 3G). The example bottom surface 222 of FIG. 3B can be prepared using injection molding (see FIG. 5A), machining, and/or any other appropriate process.

FIG. 3C shows another example of controlled texture for the top surface 220 and/or bottom surface 222 of a window 202. The example surface 220, 222 includes a controlled texture 322 which includes a crosshatch pattern of repeated features 324 and 330. The features 324 and 330 may be the same as or similar to features 306 and 310 described above (e.g., any of the example features shown in the patterned textures of FIGS. 3E-G and described above). The features 324 and 330 may be straight or curved. The features 324, 330 may be machined in the surface 220, 222 and/or prepared using injection molding. The features 324, 330 may have any appropriate depth and/or height. As an example, the features 324, 330 may have a depth/height of about 25 micrometers to about 500 micrometers (e.g., 0.001 inch to about 0.02 inch). The features 324, 330 may have any appropriate width. As an example, the features 324, 330 may have a width of about 120 micrometers to about 750 micrometers (e.g., 0.005 inch to about 0.03 inch). The distances 326, 332 between adjacent features 324, 330 may be any appropriate value. As an example, the distances 326, 332 between adjacent features 324, 330 may be in a range from about 120 micrometers to about 750 micrometers (e.g., 0.005 inch to about 0.03 inch). The first set of features 324 may be at an angle 328 (e.g., 45°) relative to the direction of the width of the window 202. The second set of channel features 330 may be at an angle 334 (e.g., 90°) relative to the first set of features 324. The example surface 220, 222 of FIG. 3C may prepared by machining (e.g., CNC milling with an end having an appropriate diameter, laser machining, or the like).

The patterned features shown in FIGS. 3A-G are examples only. A window 202 can generally include any combination of the surfaces 220, 222 and/or textures 302, 304, 320, 322, 340, 350, 360, 370 described above with respect to FIGS. 3A-G. For example, a top surface 220 may have the roughened texture 302 of FIG. 3A and/or the repeated patterned texture 322 of FIG. 3C. Meanwhile, a bottom surface 222 may include the patterned texture 304 of FIG. 3B or the patterned texture 322 of FIG. 3C. This disclosure contemplates the use of any other appropriate patterned feature and/or texture for facilitating the reliable preparation of windows 202 with consistent light transmission properties and long lifetimes for monitoring CMP processes. For example, a bottom surface 222 may include one or more of a wavy texture or the like.

The depth of machined and/or injection molded features (see FIGS. 3B, 3C, 3E, 3F, and 3G) may be adjusted to improve light transmission properties for improved monitoring CMP processes (e.g., for endpoint detection). For instance, the depth of machined cuts to achieve the crosshatch features of FIG. 3C may be adjusted to achieve a desired light transmission at a given wavelength and/or provide a desired value of the ISRM_(max-min) characteristic described with respect to FIG. 1C above. In some embodiments, the depth/height of features is selected such that even if a portion of the surface of the window 202 is removed during a CMP process and/or if the window 202 deforms or moves during a CMP process, the light transmission properties of the window 202 remain relatively constant. For example, it may be beneficial to have a depth of cut used during machining to be greater than 120 micrometers (e.g., about 0.005 inch). In some embodiments, the depth of cut used during machining (or depth of features prepared using injection molding) may be in a range from about 25 micrometers to about 75 micrometers (e.g., 0.001 inch to 0.003 inch). In some cases, varying the depth of the cut along the surface 220, 222 being machined may provide further control over the transmission of light through the window 202.

In some embodiments, separate pieces may be prepared, such that one piece includes the top surface 220 and another piece includes the bottom surface 222. FIG. 4 illustrates such an embodiment in which a first piece 402 that includes the top surface 220 is disposed on a second piece 404 that includes the bottom surface 222 to form the window 202. The pieces 402 and 404 may be attached using an adhesive (e.g., a pressure-sensitive adhesive). In some cases, the pieces 402 and 404 may be chemically bonded to form window 202 (e.g., in the presence of heat and/or appropriate adhesion promoters). The window 202 may be disposed in or on an aperture in the pad 200 with or without an adhesive, such that light passing through the aperture also passes through the window 202. In some embodiments, the window 202 is chemically bonded to the CMP pad 200.

Example Molds for Injection Molded Windows

FIGS. 5A and 5B illustrate example molds 502 and 510 used in an injection molding device 500 to prepare surfaces 222 and 220, respectively. Mold 502 for preparing the bottom surface 222 includes an inverted representation of the textures 304, 320 described above with respect to FIG. 3B. A region 504 of the mold 502 corresponds to region with texture 304 of the bottom surface 222 of FIG. 3B. A region 506 corresponds to region with texture 320 of the bottom surface 222 of FIG. 3B. The mold 502 may include ejection pins which aid in removing the formed piece from the device 500. Mold 510 for preparing the top surface 220 of the window 202 includes an inverted representation of the surface 220 described above with respect to FIG. 3A. Mold 510 may be bead blasted to obtain an appropriately roughened texture (e.g., the randomly roughened texture 340 of FIG. 3D for texture 320 or the repeated patterned textures 350, 360, 370 of FIGS. 3E-G for texture 304). During injection molding, a single piece may be formed with both the top and bottom surfaces on opposite sides of the piece, or separate pieces may be prepared and combined as described above with respect to FIG. 4 and below with respect to FIG. 8.

Transmission of Light Through Different Windows for CMP Pads

FIGS. 6A and 6B show plots 600 and 610, respectively of the transmission of light at various wavelengths through different windows, including various control windows, which employ previous technology (controls 1-4 and controls A-B), samples with a single randomly textured surface (single texture 1-6 and single texture A-C), and the new technology disclosed herein with a controlled texture comprising repeated patterned features on at least the bottom surfaces (samples 1-4 and samples A-E). Percent transmission was measured using a Thermo-Fisher Genesys 10s UV-Vis spectrophotometer at 633 nm. As described above, this disclosure recognizes that light transmission through the window 202 should be within a threshold range 602 that ensures that the signal reaching the detector 114 (see FIG. 1B) is appropriate for reliable monitoring of CMP processes. The threshold range 602 may be less than 20%. In some cases, the threshold range may from 5% to 15% at about 630 nm. A different threshold range may be selected for a different application (e.g., depending on properties of the detector 114 (see FIG. 1A) used for in situ rate monitoring/endpoint detection), and the textures of the top surface 220 and bottom surface 222 may be adjusted accordingly.

As also recognized herein, it is important to have a consistently low value of the ISRM_(max-min) characteristic that does not change significantly over the life of a CMP pad. This cannot be achieved by achieving the target transmission range alone but also requires further improved properties of the CMP window. For example, at least certain of the control windows shown in FIGS. 6A and 6B display a light transmission in the target transmission. However, these control windows, which employ previous technology, fail to provide a consistently low ISRM_(max-min) characteristic over the life of a CMP pad, as described further with respect to FIG. 6C and TABLE 1 below.

FIG. 6C shows values of the ISRM_(max-min) characteristic for two previously available CMP windows (Controls 1 and 2) and two CMP windows with the repeated patterned textures of this disclosure (New windows 1 and 2). After CMP pad break-in (e.g., after conditioning as described with respect to FIG. 1A above), the new windows have lower values of the ISRM_(max-min) characteristic than the control windows. At or near the end of the pad life (e.g., after the CMP pad can no longer be used for CMP processes, such as after a top layer of the pad is fully removed and/or after about 0.5 millimeters of the top surface of the CMP pad and/or CMP window are removed), the control windows have substantially increased values of the ISRM_(max-min) characteristic. The end of the pad life may be defined as when the pad wear was such that 20% of the initial groove depth was remaining. The ISRM_(max-min) characteristic of Control 1 increases by about 190% (i.e., from 0.75 to 1.105). The ISRM_(max-min) characteristic of Control 2 increases by about 160% (i.e., from 0.75 to 1.955). In contrast, the ISRM_(max-min) characteristic of the new windows does not increase significantly from post break-in to the end of pad life. The ISRM_(max-min) characteristic of New Window 1 increases by about 5% (i.e., from 0.275 to 0.29). The ISRM_(max-min) characteristic of New Window 1 increases by about 90% (i.e., from 0.15 to 0.21). The values of the ISRM_(max-min) characteristic for the new windows also remains in a target range of less than 0.5% (or lower) throughout the pad's life.

The new window samples (samples 1-4 and A-D of FIGS. 6A and 6B and new windows 1 and 2 of FIG. 6C) with the controlled texture described in this disclosure not only have percent transmission values within this threshold range 602 but also have repeated patterned features (see, e.g., FIGS. 3A-G and the corresponding description above). These samples have more reliable and reproducible transmission properties and ISRM_(max-min) characteristics than can be achieved using previous technology. Thus, the new window samples described in this disclosure perform better than previous windows for CMP pads and can be prepared using more reliable and cost-effective processes (see FIGS. 7-9).

Methods of Preparing Textured Surfaces and Textured Windows for CMP Pads

FIG. 7 illustrates a process 700 for preparing a window 202. The process 700 generally facilitates the preparation of a window 202 with improved light transmission properties and increased lifetimes. Process 700 may begin at step 702 where a first textured surface 220 is prepared. The first textured surface 220 may be prepared using injection molding and/or any appropriate machining process (e.g., CNC machining with an endmill, laser machining, or the like). For example, an uncured thermoset material may be introduced into an injection molding machine to contact a mold (e.g., the mold 510 of FIG. 5B) to prepare the first textured surface 220. In some embodiments, a piece may be obtained (e.g., a commercially available transparent piece may be obtained), and one surface of the piece may be machined or treated with an abrasive to prepare the first textured surface 220.

At step 704, a second surface 222 is prepared on the opposite face of the piece from step 702. If injection molding was used to prepare the first textured surface 220 at step 702, the second textured surface 222 may be prepared during the same injection molding process (e.g., using mold 502 of FIG. 5A as the mold for the second textured surface 222). As another example, the second textured surface 222 may prepared using any appropriate machining process.

At step 706, the window 202 from step 704 is disposed in or on an aperture in a CMP pad 200, such that light passing through the aperture also passes through the window 202. The window 202 may be attached to the CMP pad 200 using an adhesive (e.g., a pressure-sensitive adhesive) or without an adhesive (e.g., using ultrasonic welding or any other appropriate technique). For example, the window 202 may be attached with an adhesive or via welding to a top pad portion or a subpad portion of the CMP pad 200. In some embodiments, the window 202 is chemically bonded to the CMP pad 200 (e.g., in the presence of heat and/or appropriate adhesion promoters).

As an illustration of the attachment of the inventive window to the polishing pad, the following procedure may be used. A recess or pocket may be formed in the top pad using CNC or like method. The dimension of the recess will align with the dimension of the window to be installed. The dimensions of the recess are thus variable, but the recess does not extend the entire thickness of the top pad. In other words, the recess does not form an aperture through the top pad. A subpad is then laminated, or otherwise adhered to the top pad. A hole is then punched through the top pad and the sub pad at the recess, whereas the dimension of the hole is such that a ledge is formed by the remaining top pad material and the underlying subpad material. The window is then installed in the recess, on top of the hole, forming an optical detection port, or window, in the pad. This illustrates one embodiment of installing an inventive window into a polishing pad.

FIG. 8 illustrates an example process 800 for preparing a window 202 by combining a first piece with a first textured surface 220 (e.g., piece 402 of FIG. 4) and a second piece (e.g., piece 404 of FIG. 4) with a second textured surface 222. The process 800 generally facilitates the preparation of a window 202 with improved light transmission properties and increased lifetimes. Process 800 may begin at step 802 where a first window piece (e.g., piece 402 of FIG. 4) with a first textured surface 220 is prepared. The first piece may be prepared using injection molding and/or any appropriate machining (e.g., CNC machining with an endmill). For example, an uncured thermoset material may be introduced into an injection molding machine to contact a mold (e.g., the mold 510 of FIG. 5B) to prepare the first piece using injection molding. The piece may be further machined to generate any further desired texture features. The method 900 described below with respect to FIG. 9 may be used to prepare the first piece using injection molding and/or machining. In some embodiments, a piece may be obtained (e.g., a commercially available transparent piece may be obtained), and the piece may be machined to prepare the first textured surface 220.

At step 804, a second window piece (e.g., piece 404 of FIG. 4) with a second textured surface 222 is prepared. The second piece with the second textured surface 222 may be prepared using the same or a similar approach to that described above for preparation of the first piece with the first textured surface 220 at step 802. The second piece may be prepared using injection molding and/or any appropriate machining (e.g., CNC machining with an endmill). For example, an uncured thermoset material may be introduced into an injection molding machine to contact a mold (e.g., the mold 502 of FIG. 5A) to prepare the second piece using injection molding. The second piece may be further machined to generate any further desired texture features on the second textured surface 222. The method 900 described below with respect to FIG. 9 may be used to prepare the second piece using injection molding and/or machining. In some embodiments, a piece may be obtained (e.g., a commercially available transparent piece may be obtained), and the piece may be machined to prepare the second textured surface 222.

At step 806, the first piece with the first textured surface 220 is disposed on (e.g., attached to) the second piece with the second textured surface 222. The two pieces are generally combined such that the first surface 220 is exposed and faces a first direction (e.g., towards the top of the window 202) and the second surface 222 is exposed and faces a second direction opposite the first direction (e.g., towards the bottom of the window 202). The first and second pieces may be attached using an adhesive (e.g., a pressure-sensitive adhesive) or without an adhesive to form the window 202. In some cases, the first and second pieces may be chemically bonded to form window 202 (e.g., in the presence of heat and/or appropriate adhesion promoters).

At step 808, the window 202 from step 806 is disposed in or on an aperture in a CMP pad 200, such that light passing through the aperture also passes through the window 202. The window 202 may be attached to the CMP pad 200 using an adhesive (e.g., a pressure-sensitive adhesive) or without an adhesive (e.g., using ultrasonic welding or any other appropriate technique). For example, the window 202 may be attached with an adhesive or via welding to a top pad portion or a subpad portion of the CMP pad 200. In some embodiments, the window 202 is chemically bonded to the CMP pad 200 (e.g., in the presence of heat and/or appropriate adhesion promoters).

FIG. 9 illustrates an example method 900 for preparing a controlled textured surface (e.g., a surface 220, 222 described above). The method 900 may begin at step 902 where injection molding is performed to prepare a piece (e.g., a piece 402, 404 of FIG. 4) with a textured surface 220, 222. For example, an uncured thermoset material may be introduced into an injection molding machine to contact a mold (e.g., a mold 502, 510 of FIGS. 5A, 5B) to prepare the piece using injection molding. At step 904, a determination is made of whether the piece textured surface 220, 222 should have a further texture features, such as one of the patterned features illustrated in FIGS. 3A, 3B, 3C, or the like. If additional features are not needed, the process 900 is complete. However, if further features are needed, the further features are machined (e.g., using CNC machining with an endmill) at step 906.

EXAMPLE

This example demonstrates the performance of the inventive windows compared to windows not having a controlled texture comprising repeated pattern features (a repeated patterned texture).

Four different windows were evaluated for performance using the change in the ISRM_(max-min) characteristic measured after a break-in period and again at the end of the pad life. The end of the pad life is defined as when the pad wear was such that 20% of the initial groove depth was remaining. The four windows were A) IC1010 pad with window, a hard thermoset polyurethane with a hardness of about 70 Shore D, commercially available from Rohm and Haas Electronic Materials; B) E6088 pad with a soft thermoplastic polyurethane having a hardness of about 55 Shore D, commercially available from CMC Materials Inc.; C) E6088 pad with an inventive window having controlled, repeated pattern texture from CNC machining, a thermoplastic polyurethane having a hardness of 75 Shore D; and D) E6088 pad having an inventive window with controlled, repeated pattern texture from injection molding, a thermoplastic polyurethane having a hardness of 75 Shore D. Windows A) and B) did not have a controlled texture surface comprising repeated patterned features.

All pads described above were given identical break-in period comprising 30 minutes of conditioning with deionized water, using an A2813 conditioner from 3M, at 5 lb. downforce. Following the break-in period, the ISRM_(max-min) characteristic was measured under identical conditions for each of the windows while polishing copper wafers, using a copper CMP polishing slurry, at 2.5 psi downforce and 80 rpm platen speed. The end-point detection was started about 20 seconds after the start of the polishing, and the record consisted of about 60 traces. The ISRM_(max-min) characteristic was calculated for the four windows and is shown below in the Table. A second ISRM_(max-min) characteristic measurement was taken on the same pads at the end of pad life. The ISRM_(max-min) characteristic was again calculated and is shown below in TABLE 1.

TABLE 1 Values of ISRM_(max-min) characteristic for example CMP windows ISRM_(max-min) (%) ISRM_(max-min) (%) Window Post Break-in End of Pad Life Change A 0.380 1.105 0.725% B 0.750 1.955 1.205% C 0.275 0.290 0.015% D 0.150 0.210 0.060%

As can be seen in the Table above, the change in ISRM_(max-min) from the post break-in period to the end of pad life was dramatically different for the inventive windows when compared to the windows of commercial pads.

Modifications, additions, or omissions may be made to the systems, apparatuses, and methods described herein. The components of the systems and apparatuses may be integrated or separated. Moreover, the operations of the systems and apparatuses may be performed by more, fewer, or other components. The methods may include more, fewer, or other steps. Additionally, steps may be performed in any suitable order. Additionally, operations of the systems and apparatuses may be performed using any suitable logic. As used in this document, “each” refers to each member of a set or each member of a subset of a set.

Herein, “or” is inclusive and not exclusive, unless expressly indicated otherwise or indicated otherwise by context. Therefore, herein, “A or B” means “A, B, or both,” unless expressly indicated otherwise or indicated otherwise by context. Moreover, “and” is both joint and several, unless expressly indicated otherwise or indicated otherwise by context. Therefore, herein, “A and B” means “A and B, jointly or severally,” unless expressly indicated otherwise or indicated otherwise by context.

The scope of this disclosure encompasses all changes, substitutions, variations, alterations, and modifications to the example embodiments described or illustrated herein that a person having ordinary skill in the art would comprehend. The scope of this disclosure is not limited to the example embodiments described or illustrated herein. Moreover, although this disclosure describes and illustrates respective embodiments herein as including particular components, elements, feature, functions, operations, or steps, any of these embodiments may include any combination or permutation of any of the components, elements, features, functions, operations, or steps described or illustrated anywhere herein that a person having ordinary skill in the art would comprehend. Furthermore, reference in the appended claims to an apparatus or system or a component of an apparatus or system being adapted to, arranged to, capable of, configured to, enabled to, operable to, or operative to perform a particular function encompasses that apparatus, system, component, whether or not it or that particular function is activated, turned on, or unlocked, as long as that apparatus, system, or component is so adapted, arranged, capable, configured, enabled, operable, or operative. Additionally, although this disclosure describes or illustrates particular embodiments as providing particular advantages, particular embodiments may provide none, some, or all of these advantages.

The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better explain the disclosure and does not pose a limitation on the scope of claims. 

1. A window for a chemical mechanical planarization (CMP) pad, the window comprising a material transmissive to light, wherein a first surface of the window has a controlled texture surface comprising repeated patterned features.
 2. The window of claim 1, wherein the first surface corresponds to a bottom surface of the CMP pad which does not contact a substrate being planarized during a CMP process using the CMP pad.
 3. The window of claim 1, wherein the repeated patterned features are configured to diffuse light passing through the window.
 4. The window of claim 1, wherein the repeated patterned features comprise regularly spaced raised features.
 5. The window of claim 4, wherein the repeated patterned features comprise a crosshatch pattern.
 6. The window of claim 4, wherein the repeated patterned features comprise rounded ridges.
 7. The window of claim 5, wherein the rounded ridges are randomly roughened.
 8. The window of claim 2, wherein a second surface opposite the first surface corresponds to a top surface, the top surface is capable of contacting a substrate being planarized during a CMP process using the CMP pad, wherein the top surface has a roughened texture corresponding to an average surface roughness in a range from about 1 to about 50 micrometers.
 9. The window of claim 1, wherein: the window has a width and length that is less than the width, wherein the width and the length characterize physical dimensions of the window; and the repeated patterned features comprise: a first set of regularly spaced raised features parallel to a direction of the width of the window; and a second set of regularly spaced raised lines at an angle relative to the first set of regularly spaced raised features.
 10. The window of claim 1, wherein: the window has a width and length that is less than the width wherein the width and the length characterize physical dimensions of the window; and the repeated patterned features comprise a crosshatch texture comprising: a first set of regularly spaced features in the first surface at a first angle relative to a direction of the width of the window; and a second set of regularly spaced features in the first surface at a second angle relative to the first set of lines, wherein the first angle is different than the second angle.
 11. A chemical mechanical planarization (CMP) pad comprising: a top surface which contacts a substrate being planarized during a CMP process using the CMP pad; a bottom surface opposite the top surface; and a window that allows light to pass between a top side of the CMP pad associated with the top surface and a bottom side of the CMP pad associated with the bottom surface, the window comprising a material transmissive to light, wherein a first surface of the window has repeated patterned features.
 12. The CMP pad of claim 11, wherein the first surface of the window faces the same direction as the bottom surface of the CMP pad which does not contact the substrate being planarized during a CMP process using the CMP pad.
 13. The CMP pad of claim 11, wherein the repeated patterned features are configured to diffuse light passing through the window.
 14. The CMP pad of claim 11, wherein the repeated patterned features comprise regularly spaced raised features.
 15. The CMP pad of claim 11, wherein the repeated patterned features comprise a crosshatch pattern.
 16. The CMP pad of claim 11, wherein the repeated patterned features comprise rounded ridges.
 17. The CMP pad of claim 16, wherein the rounded ridges are randomly roughened.
 18. The CMP pad of claim 11, wherein a second surface opposite the first surface corresponds to a top surface, the top surface is capable of contacting a substrate being planarized during a CMP process using the CMP pad, wherein the top surface has a roughened texture corresponding to an average surface roughness in a range from about 1 to about 50 micrometers.
 19. The CMP pad of claim 11, wherein: the window has a width and length that is less than the width, wherein the width and the length characterize physical dimensions of the window; and the repeated patterned features comprise: a first set of regularly spaced raised features parallel to a direction of the width of the window; and a second set of regularly spaced raised lines at an angle relative to the first set of regularly spaced raised features.
 20. The CMP pad of claim 11, wherein: the window has a width and length that is less than the width wherein the width and the length characterize physical dimensions of the window; and the repeated patterned features comprise a crosshatch texture comprising: a first set of regularly spaced features in the first surface at a first angle relative to a direction of the width of the window; and a second set of regularly spaced features in the first surface at a second angle relative to the first set of lines, wherein the first angle is different than the second angle. a window as disclosed herein. 